Slurry process for coating particulate material upon a surface

ABSTRACT

A process for coating a flatlike, concave, nonporous surface, such as a surface of a viewing window for a cathode-ray tube, with particulate material comprising the steps in the following order: A. rotating the surface and dispensing onto the central portion of the surface a puddle of slurry comprised of a particulate material and a liquid vehicle, B. rotating the surface and spreading the puddle outwardly over the surface to the margins thereof, C. further rotating the surface and spreading the puddle inwardly over the surface, D. and further rotating the surface and spreading the puddle outwardly over the surface to the margins thereof.

Paten ted April 4, 1972 3,653,941

SPREAD OUTWARDLY Barry B. Bell and Wellington E. Pederson BYge AT TORNE Y United States Patent Bell et a].

[451 Apr. 4, 1972 [54] SLURRY PROCESS FOR COATING PARTICULATE MATERIAL UPON A SURFACE [72] Inventors: Barry Bernard Bell; Wellington Edward Pederson, both of Lancaster, Pa.

[73] Assignee: RCA Corporation [22] Filed: Mar. 2, 1970 [21] Appl. No.: 15,672

[52] U.S.Cl. ..l17/33.5C,117/101,117/102R [51] lnt.'Cl..; ..H01j3l/20 [58] FieldofSearch ..ll7/33.5,101;96/36.1

[56] References Cited UNITED STATES PATENTS 3,467,059 9/1969 Korner et al. ..l17/l0l X Primary Examiner-Alfred L. Leavitt Assistant Examiner-Wayne F. Cyron Att0rney-Glenn H. Bruestle [57] ABSTRACT A process for coating a flatlike, concave, nonporous surface, such as a surface of a viewing window for a cathode-ray tube, with particulate material comprising the steps in the following order:

7 Claims, 3 Drawing Figures SLURRY PROCESS FOR COATING PARTICULATE MATERIAL UPON A SURFACE BACKGROUND OF THE INVENTION This invention relates to a novel process for coating particulate material upon a flatlike, concave, nonporous surface. The novel process is particularly applied to the coating of phosphor particles on the inner surface of the viewing window of a cathode-ray tube, for example, a color television picture tube.

In one method of making a phosphor screen for a color television picture tube, a slurry is prepared which includes phosphor powder, a binder such as polyvinyl alcohol, a sensitizer for the binder such as ammonium dichromate, and a liquid vehicle such as water. In a previous factory process, as practiced on a semiautomated machine, a puddle of phosphor slurry is dispensed into the central portion of the inner surface of a slowly rotating viewing window, which is part of the faceplate panel. The window is rotated and tilted to spread the slurry puddle outwardly to the margins of the window and thereby coating the slurry over the entire window surface. During the spreading step, the coated slurry circulates over the surface and some of the particles therein settle on the window surface. The excess slurry and excess vehicle are then removed, as by rapidly spinning the panel to sling the excess slurry from the panel. With ordinary slurry coating factory practice, average dry phosphor screen weights of less than 2.50 mg/cm and usually about 1.75 to 2.25 mg/cm are obtained, while maintaining relatively uniform screen weights from the center to the edge of the window surface, and from panel to panel. It is desirable to increase the screen weight to above about 2.50 mg/cm and preferably about 2.75 to 3.00 mglcm and to decrease the porosity of the coating, still maintaining adequate uniformity from the center to the edge of the window, and from panel to panel.

SUMMARY OF THE INVENTION In the novel process, the surface to be coated is rotated, a puddle of slurry is dispensed into the central portion of the surface, and the puddle is spread outwardly over the surface to the margins of the surface as in the previous process. Then, the puddle is spread inwardly over the surface towards the enter of the rotating panel. The puddle is again spread outwardly over the surface to the margins thereof. The slurry may be spread inwardly and outwardly as many times as desired over the rotating surface and, then, after the final cycle, any excess slurry is removed.

By spreading the slurry inwardly and outwardly over the surface (after the initial outward spreading of slurry), additional time is allowed for particles in the slurry to settle upon the surface. The structure of the ultimate coating is improved, porosity of the coating is reduced, and higher coating weights with relatively uniform screen weights from the center to the edge are readily obtained. A color television picture tube with a phosphor screen prepared by the novel process exhibits high light output for the screen.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a faceplate panel showing the axis of rotation.

FIG. 2 is a plan view of the inside surface of a faceplate panel illustrating some factors involved in the novel process.

FIG. 3 is a series of four plan views of the inside surface of a faceplate panel illustrating the steps in the novel process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments are described with respect to coating the inner surface of the viewing window of a cathoderay tube, particularly a color television picture tube. In this process, each of three phosphors is incorporated into a separate slurry, coated on the inner surface of the panel, and then processed to produce a phosphor dot pattern. Apparatus for carrying out the coating step are disclosed in the prior art; for example, US. Pat. Nos. 2,902,973 to M. R. Weingarten; 3,319,759 to T. J. Hajduk et al; 3,376,153 to J. P. Fiore; 3,364,054 to M. R. Weingarten; and 3,467,059 to R. L. Komer et al.

In each of the prior apparatus, a faceplate panel is held in a work holder which is rotated and tilted to carry out the process steps. The work holder may move from station to station where the various process steps in the fabrication process are carried out. As part of this fabrication, a puddle of slurry is dispensed into the panel and the work holder is made to rotate and tilt according to a prescribed program for the purpose of producing a coating of phosphor on the surface of the view window of the panel. In prior processes, the rate of rotation and the angle of tilt of the rotation axis are adjusted to cause a puddle of phosphor slurry to spiral outwardly around the inner surface of the viewing window until the entire surface has been covered. Then, the excess slurry material is removed. The novel process may be carried out in the same or similar apparatus with an adjustment in the program of rotation rate, and the angle of tilt of the rotation axis and the cycle time to cause the puddle to spread outwardly, then back inwardly, then back outwardly again over the desired time period. In the preferred forms of the invention, the program for coating is carried out over an additional station when process is practiced on automated apparatus.

Some principles that apply to the novel process are illustrated in FIGS. 1 and 2. A typical faceplate panel 21 is a unitary glass structure comprised of a viewing window 23 which is generally concave on the inside, flatlike and nonporous, having sidewalls 25 which rise along the margins of the viewing window 23 and terminate with a seal edge 27, which subsequently is sealed to the seal edge on the funnel portion of the tube. These figures show a rectangular window 23, but the window may have any other shape, for example, round. When the panel 21 is mounted in the work holder of the coating machine, it is made to rotate about an axis 29 which is normal to and passes through the central portion (a generalized region) of the inner surface of the window 23. In this application, all rotations are clockwise facing the coated surface, although counter clockwise rotation may be used. The axis of rotation 29 may be tilted from vertical (indicated by the line 30) by a tilt angle 31.

A puddle 33 of slurry is dispensed into the central portion of the window 23 and spread over the inside window surface by the rotation and tilting of the panel 21. The puddle 33 referred to herein is a quantity of slurry which moves overthe window surface as an entity having a leading edge 34 and leaving a trail 35 of material behind. The puddle 33 may pick up material trails from previous trails during its travel over the surface. The material left behind generally moves downwardly, as indicated by the arrows 32, by gravity so that the slurry is constantly distributing itself over the surface, obliterating the edges of previous trails, and to some extent draining back into the puddle.

The tilt angle 31 of the rotation axis 29 and the speed of rotation are selected to produce the desired spreading. The rotation of the panel has the effect of carrying the coating material up the the high side of the panel and, if fast enough, down the other side. Gravity, which is enhanced by greater angles of tilt, causes the trailed material (and the puddle) to drain downwardly. A combination of these two factors is programmed to move the puddle, which is principally excess slurry, outwardly toward the margins of the window 23, then inwardly and then outwardly again. Generally, as the tilt angle 31 becomes greater, the puddle moves from the central portion to the margins of the window 23 And, as the speed of rotation is increased, the puddle moves from the central portion to the margin of the window. Thus, for example, by slowing rotational speed without changing the tilt angle 31, the puddle can be made to move inwardly. Or, by increasing the tilt angle 31 without changing the rotational speed, the puddle can be made to move outwardly.

Viscosity, specific gravity, and other characteristics affect the optimum angle 31 of the axis 29, the rotational speed and the total coating time. Some suitable phosphor slurries are described in U. S. Pat. Nos. 3,269,838 to T. A. Saulnier and 3,313,643 to P. B. Branin. Suitable slurries usually have viscosities in the range of 25 to 75 centipoises.

The steps in the novel process are illustrated by a series of four views shown in FIG. 3. In step (A), a quantity of slurry is dispensed into the central portion of the inner surface of the window 23 forming a puddle 33 which is made to rotate in that region leaving behind a trail 35. During the dispensing step or after it is completed, the puddle 33 is made to move outwardly to the margins of the window 33 as shown by the dotted spiral 37 in FIG. 3, step (B). Then in step (C), the puddle 33 is made to spread inwardly toward the central portion of the window as shown by the dotted spiral 39. In step (D), the puddle 33 is again made to spread outwardly to the margins of the window as shown by the dotted spirals 41. In this latter step, the slurry material is well spread out over the surface of the window and the leading edge 34 of the puddle takes a different shape from that shown in steps (B) and (C).

The examples of the novel process given below are compared with a previous process wherein a quantity of slurry is dispensed into the central portion of the inner surface of the viewing window with the axis of rotation tilted at about 17 from vertical. The dispensing continues for about 6 seconds with the window rotating at about 20 rpm. At the end of the dispensing, the axis is inclined to about 27 in about seconds and the speed decreased to about 11 rpm. This angle and speed are continued for about 11 seconds and then the excess slurry is removed. During the prior process, the dispensed slurry forms as a puddle in the central portion of the window and then spreads outwardly to the margins as the rotation and tilting proceed.

To compare the coating weights or screen weights of phosphor deposited by the various processes, the following procedure was used. The coating is dried and then a template of known size (usually about 5 cm. by 10 cm.) is pressed against the coating, and the area surrounding the template is scraped away. This defines the coating area to be sampled. Then, the template is removed and the coating in the defined area is scraped off and collected in a tared (previously weighed) aluminum pan. The scraped material and pan are then heated until the volatile matter is removed. Then, the pan and its contents are weighed. The weight difference is the weight of phosphor coated on the defined area. This weight difference divided by the area is the density of the phosphor coating deposited and is referred to as the coating weight or screen weight.

EXAMPLE I In this example, the inner surface of the viewing window of a faceplate for a 23 inch V color television picture tube is coated with a green-emitting phosphor. Referring again to FIG. 3, in step (A) about 130 cc of phosphor slurry containing blue-emitting phosphor particles, dichromated polyvinyl alcohol, and water are dispensed into the central portion of the viewing window which is rotating at about 11 rpm about an axis of rotation which is tilted at about 5 from vertical. Steps (A) and (B) overlap in time. During step (B), the tilt angle 31 of the rotation axis is increased gradually from about 5 to about 25 over a period of about seconds and the rotation is decreased to about 7 rpm. The change in tilt angle and rotational speed causes the puddle 23 to spiral over the surface and spread outwardly to the margins of the window. Then, the tilt angle 31 is brought back to about 5 and the rotation is increased to about 11 rpm, causing the puddle 33 to spiral and spread inwardly as shown in step (C). The tilt angle 31 is again gradually increased from about 5 to about and the speed of rotation reduced from about I 1 rpm to about 7 rpm over a period of about 17 seconds, causing the puddle to again spiral and spread outwardly to the margins of the window 23. Upon completion of the above process cycle, the panel is rotated at about 175 rpm for 15 to 20 seconds causing the excess slurry material to move outwardly by centrifugal force, upward along the corners of the sidewall and off the panel in a manner described in the above-cited Weingarten U.S. Pat. No. 3,364,054.

In one test which may be considered representative, the coating weight was about 2.97 mg/cm near the center of the window and 2.98 mg/cm near the margin of the window. This is to be compared with a weight of about 2.50 mg/cm near the center and also near the margin of the window for the same slurry applied by the previous process described above. After incorporation of these coatings into a cathode-ray tube, the luminescent screens prepared by the novel process required about 8 percent less gun current to excite the screen to the same brightness level as compared with similar screens prepared by the previous process.

EXAMPLE 2 The same procedure described in Example I was followed except that blue-emitting phosphor was used in the slurry in place of green-emitting phosphor. In this test, the screen weight was about 3.86 mg/cm near the center of the window and about 3.60. mg/cm near the margin of the window, as compared with screen weights of about 2.36 and 2.50 mg/cm respectively for the same slurry applied by the previous process described above.

EXAMPLE 3 In this example, green-emitting phosphor is coated upon the inner surface of the viewing window for a 23-inch V color television picture tube. About cc of a slurry containing green-emitting phosphor particles, dichromated polyvinyl alcohol and water are dispensed into the central portion of the viewing window of a faceplate panel forming a puddle 33, as shown in step (A) of FIG. 3. During dispensing, the panel is rotating at about 10 rpm and the rotational axis 29 is tilted at a tilt angle 31 of about 15. Near the end of the dispensing step, the speed of rotation is reduced from 10 rpm to about 6 rpm and the tilt angle 31 of the rotational axis 29 is increased from about l5 to about 25, causing the puddle 33 to spiral over the window surface and to spread outwardly toward the margins thereof as shown in step (B). The axis 29 is kept tilted at about 25 and the rotational speed is slowed to less than 5 rpm, causing the puddle 33 to spiral and spread inwardly as shown in step (C). Slowing the rotation causes the puddle material to be carried up to the high side and to slide down into the central portion of the window. At faster speeds the slurry is carried up to the high side and back down without spreading toward the central portion of the window. Finally, in step (D) the axis 29 of rotation is retained at a tilt angle 31 of about 27, but the speed of rotation is increased to about 8 rpm causing the puddle 33 to again spread outwardly. On an extended run, the screen weights averaged about 2.74 mg/cm as compared with a similar run using the prior process described above which produced average screen weights of about 2.31 mglcm Gun currents (normalized for purposes of comparison) required to produce 8 ft. lamberts of white light from the television picture tube were reduced more than 10 percent using the novel process on the average.

We claim:

1. A process for coating a cathode-ray-tube viewing-window surface with particulate material comprising the steps in the following order:

' A. rotating said surface about an axis that is substantially normal to and passes through the central portion of said surface, and dispensing onto said central portion of said surface a puddle of slurry comprised of said particulate material mixed with a liquid vehicle, said slurry having a viscosity of about 25 to 75 centipoises,

B. rotating said surface and spreading said puddle outwardly over said surface to the margins thereof,

C. further rotating said surface and spreading said puddle inwardly over said surface towards the central portion thereof,

D. and further rotating said surface and spreading said puddle outwardly over said surface to the margins thereof.

2. The process defined in claim 1 wherein step (B) includes gradually tilting said axis away from vertical, step (C) includes gradually tilting said axis towards vertical and step (D) includes gradually tilting said axis away from the vertical.

3. The process defined in claim 1 wherein step (B) includes gradually tilting said axis away from vertical to a predeter mined angle and speed of rotation of said surface, step (C) includes maintaining said axis at said predetermined angle and decreasing the speed of rotation of said surface and step (D) includes maintaining said axis at substantially said predetermined angle and increasing the speed of rotation of said surface.

4. A process for coating particulate phosphor material on the inner surface of a viewing window of a cathode-ray tube comprising:

A. rotating said surface about an axis that is substantially normal to and passes through the central portion of said surface, and dispensing onto said central portion of said surface a puddle of slurry comprised of phosphor particles, a binder, and a liquid vehicle, said slurry having a viscosity of about 25 to 75 centipoises,

B. rotating said surface about said axis and spreading said puddle outwardly over said surface to the margins thereof,

C. further rotating said surface about said axis and spreading said puddle inwardly over said surface towards the central portion thereof,

D. further rotating said surface about said axis and spreading said puddle outwardly over said surface to the margins thereof,

E. and then removing any excess slurry material from said surface.

5. The process defined in claim 4 wherein, at the beginning of step (A) said window is rotating at about 12 rpm and said axis is about 5 from vertical, then for step (B) the axis is gradually tilted to about 25 from vertical and the rotation is decreased to about-7 rpm, then for step (C) the axis is raised to about 5 from vertical and the rotation increased to about 12 rpm, then for step (D) the axis is gradually tilted to about 25 from vertical and the rotation is decreased to about 7 rpm.

6. The process defined in claim 4 wherein, at the beginning of step (A) said window is rotating at about 10 rpm and said axis is about 15 from vertical, then for step (B) the axis is gradually tilted to about 25 from vertical and the rotation is decreased to less than about 6 rpm, then for step (C) with the axis still at about 25 from vertical the rotation is decreased to less than 5 rpm, then for step (D) the axis is gradually tilted to about 27 from vertical and the rotation is increased to about 9 rpm.

7. The method defined in claim 4 wherein step (E) is conducted by increasing the rotation of said panel to more than rpm, whereby to flow said excess material outwardly and to sling said material away from said surface. 

2. The process defined in claim 1 wherein step (B) includes gradually tilting said axis away from vertical, step (C) includes gradually tilting said axis towards vertical and step (D) includes gradually tilting said axis away from the vertical.
 3. The process defined in claim 1 wherein step (B) includes gradually tilting said axis away from vertical to a predetermined angle and speed of rotation of said surface, step (C) includes maintaining said axis at said predetermined angle and decreasing the speed of rotation of said surface and step (D) includes maintaining said axis at substantially said predetermined angle and increasing the speed of rotation of said surface.
 4. A process for coating particulate phosphor material on the inner surface of a viewing window of a cathode-ray tube comprising: A. rotating said surface about an axis that is substantially normal to and passes through the central portion of said surface, and dispensing onto said central portion of said surface a puddle of slurry comprised of phosphor particles, a binder, and a liquid vehicle, said slurry having a viscosity of about 25 to 75 centipoises, B. rotating said surface about said axis and spreading said puddle outwardly over said surface to the margins thereof, C. further rotating said surface about said axis and spreading said puddle inwardly over said surface towards the central portion thereof, D. further rotating said surface about said axis and spreading said puddle outwardly over said surface to the margins thereof, E. and then removing any excess slurry material from said surface.
 5. The process defined in claim 4 wherein, at the beginning of step (A) said window is rotating at about 12 rpm and said axis is about 5* from vertical, then for step (B) the axis is gradually tilted to about 25* from vertical and the rotation is decreased to about 7 rpm, then for step (C) the axis is raised to about 5* from vertical and the rotation increased to about 12 rpm, then for step (D) the axis is gradually tilted to about 25* from vertical and the rotation is decreased to about 7 rpm.
 6. The process defined in claim 4 wherein, at the beginning of step (A) said window is rotating at about 10 rpm and said axis is about 15* from vertical, then for step (B) the axis is gradually tilted to about 25* from vertical and the rotation is decreased to less than about 6 rpm, then for step (C) with the axis still at about 25* from vertical the rotation is decreased to less than 5 rpm, then for step (D) the axis is gradually tilted to about 27* from vertical and the rotation is increased to about 9 rpm.
 7. The method defined in claim 4 wherein step (E) is conducted by increasing the rotation of said panel to more than 150 rpm, whereby to flow said excess material outwardly and to sling said material away from said surface. 